Will be used/needed in the upcoming nfnetlink/nftables support. This follows existing model where kernel uapi headers are cached locally.
The files in this directory are copied from current kernel master (b06ed1e7a2fa9b636f368a9e97c3c8877623f8b2) or WireGuard master (8416093498ac2c754536dad4757c5d86c9ba8809), and the following modifications are applied: - btrfs.h: drop '__user' attributes - if.h: drop '#include <linux/compiler.h>' and '__user' attributes